On the first 3 quizzes of his geometry class, Luis got an average score of 85. What does he need on the next quiz to have an overall average of 87?
Explanation: Let his score on the next quiz be $x$ The sum of all of his scores is then $3 \cdot 85 + x$ The same sum must also be equal to $4 \cdot 87$ Solve: $x = 4 \cdot 87 - 3 \cdot 85 = 93$.